Monoammonium phosphate, hereafter called MAP, is commonly made buy neutralizing orthophosphoric acid (H.sub.3 PO.sub.4) with ammonia (NH.sub.3) at an N/P mole ratio of 1.0, as follows: EQU NH.sub.3 + H.sub.3 PO.sub.4 .fwdarw.NH.sub.4 H.sub.2 PO.sub.4 (.DELTA. H) = -32 kcal/mol.
This reaction is generally carried out in the presence of sufficient water to make a mobile fluid of the reaction medium. Since more heat is evolved than necessary to raise the resultant mixture to its boiling point, excess heat is utilized to drive off some of the water. Where concentrated acid feedstock is used, water must be added if formation of polyphosphates if to be avoided.
The more important methods of making MAP are summarized below, but not necessarily in order of commercial significance. Pertinent patents are listed in Table IV.
The slurry process developed by Achorn et al. U.S. Pat. No. 3,153,574, has been in commercial use for more than fifteen years. Crude MAP by this process is commonly known as 11-48-0 in the fertilizer industry and is generally made from wet-process merchant-grade acid (54% P.sub.2 O.sub.5) and ammonia. The limited solubility of MAP requires a relatively large percentage of water to keep the viscosity of the resulting slurry low. The hot slurry is mixed in a drum granulator with a high rate of recycle of previously granulated MAP. Recycle rates of 20:1 or more are not exceptional for this material but are usually 5:1 at the lowest. The large quantity of water necessary for granulation requires that the granular MAP be dried in a gas-fired rotary dryer. Granulated MAP from this process is too dense, making it undesirable for further ammoniation in fertilizer processes because of its low reactivity. In recent years, several companies have developed proprietary spray processes to avoid the troublesome granulation process. The Hemsley et al method, U.S. Pat. No. 3,415,638, uses a pressurized neutralizer opening at 30 pounds per square inch gauge to neutralize merchant-grade phosphoric acid with ammonia to a 1.0 N/P mole ratio in a single stage. The resultant slurry is sprayed into a void tower and collects as tiny prills at the bottom of the tower, or optionall (Example IV) is sprayed onto a moving belt.
Kearns, U.S. Pat. No. 3,419,378, neutralizes wet-process acid with ammonia in a single-stage reactor. The mostly liquid reacting mixture emerges instantly from a nozzle and is sprayed into a tower. Excess ammonia is required to complete the ammoniation to 1.0 N/P mole ratio, and the waste ammonia must be recovered from the spray tower.
Scottish Agricultural Industries developed a process called "Phosai", British Pat. No. 951,476, to neutralize phosphoric acid to a mole ratio of 1.4 N/P in a pre-neutralizer. The resultant slurry is pumped to a pugmill where the slurry is acidified to 1.0 N/P ratio with additional phosphoric acid. This reaction is carried out in the presence of sufficient recycle to keep the mass of materials in good mechanical condition. The granules are dried in a rotary dryer.
The Swedish firm of A. B. Forenade Superfosfatfabrik developed a process in which phosphoric acid was sprayed into an ammonia atmosphere. The heat of reaction is sufficient to evaporate the water from the ammonium phosphate particles, provided an acid of sufficient strength is used. Heat deficiencies in certain phosphoric acids used for this process are made up by adding sulfuric acid; however, the product then contains a mixture of ammonium phosphate and ammonium sulfate. The product from this process is such a fine powder that it is impractical to use in normal fertilizer granulation plants.
The reader may also be interested in U.S. Pat. Nos. 3,053,622, 3,902,342, 3,310,371, 3,502,441, 3,539,327, 2,890,934, 3,238,021, 3,019,099, 3,130,033, 3,179,496, 3,375,063, 3,730,700, and British Pat. No. 1,081,296.